Water moves in soil through a combination of gravity and capillary action. Capillary action is the force that moves water into a blotter or paper towel.
Fine-textured soil like clay has a stronger capillary pull than coarse-textured soil, but greater friction, so water moves more slowly through it. Water moves quickly through sandy soil, but there is less capillary pull. When soils of different textures are layered with a sharp interface, as when sandy loam is dumped on top of clay soil without mixing it in, water resists crossing the interface. This happens if clay is on top of sand or sand on top of clay.
If clay is on top of sand, the greater capillary pull of the clay keeps water in the clay and won’t let it enter the sand. If sand is on top of clay, the resistance of the clay to water penetration slows down its entering, and the water drains away through the sand before it enters the clay. The problem only occurs in cases where the interface between the two different textures is sharp and the textures are very different from one another.
Drainage problems in layered soil can be solved by blending the different soil textures together to eradicate the sharp interface. If the top layer is within 8 inches of the surface, this can be done simply by tilling deep enough to mix them together. In cases where the interface is too deep to till through it, dig holes through the interface, perhaps with a post-hole digger or soil auger, every few feet. Backfill the holes with the soil that was dug out, which will be a blend of the two soils. These holes create breaks in the interface through which water can drain, eliminating the layering problem.